VALVE
- BURNT
There
are many reasons for burnt out valves. Some times it is a combination of reasons
but it is generally easy to determine the major cause.
- Weak
valve springs: check valve spring tension and free length also check installed
height.
-
Low seat height:
check valve installed height and valve recession.
- Worn
valve guides: check
guide wear.( using a new valve as a gauge is the simplest method)
- Worn
valve stems:
measure valve stem in all positions of wear.(micrometer is the best method)
- Incorrectly
set tappets:
this can only be determined before strip down. (hydraulic tappet depression
should also be checked)
- Incorrectly
faced valves:
check valve face finish. (finish should be fine without any chatter or flats)
check valve for run out. Check for correct face angle. Should be 1 degree
different, to allow contact at top side of seat.
- Incorrectly
cut seat:
seat contact position on a valve should be around .1 to 1.5mm away from
the valve margin.(the centre third of the valve face )
-
Incorrect seat width:
The seat contact width is generally between 1 and1.5 mm. Generally three
angles are involved in seats. Check the seat angle is either 45 or 30 degrees,
with a top cut angle of 30 or 15 degrees and a throating angle of 70 or
0 degrees.
-
Concentricity:
The concentricity of the seat to the guide has to be checked.(using bearing
blue, or vacuum tester)
-
Incompatible fuel for engine:
LPG used without any head modifications if required, ULP where super or premium should be used.
- Incorrect
ignition timing:
driving vehicle at sustained high revs while the ignition timing is incorrect
will contribute to burnt valves. Excessive combustion temperatures will
cause exhaust valves to burn.( check base ign.timing and full advance timing.)
- Incorrect
fuel mixture:
incorrect fuel mixture can increase the combustion temperatures to a point
where sustained driving will burn valves.
- Poor
oil control:
Build up of carbon on the valve stem or seat prevents correct seating to
take place resulting in poor heat transfer from the valve head to cooling
system.
A
general over view is that some of the causes listed can produce burnt valves
on their own, while it is quite often a combination of a number of these causes
that ultimately results in the burnt out valve.
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: Every care has been taken in writing this information and procedures, but no responsibility
can be excepted for errors, omissions or misuse of this information and procedures. The
information available on this site is for your instruction only and cannot be copied for
sale, © copyright 2001
UMR Engines www.engineproblem.com.au
